1. Field of the Invention
The present invention relates generally to scale inhibitors used to prevent the formation of magnesium or calcium containing scale in water evaporation systems, boilers, water purification equipment and the like. More specifically, the present invention relates to a mixture of three scale inhibitors which produces an unexpected synergistic amount of scale prevention, when compared to the results obtained by using the single components or combinations of two of the three components.
2. Description of the Prior Art
Various types of salts which are soluble in natural waters, for example, salt water, will deposit as scale when the concentration of such salts exceeds the saturation value for the particular salt during processing, treatment or use of the water. Deposit of scale requires periodic cleaning of the equipment, leading to equipment downtime, the use of expensive cleaning chemicals, corrosion of the equipment, and the consequent labor cost associated with such cleaning. Such problems have been recognized for many years, and numerous researchers have attempted to develop systems for preventing or minimizing scale deposits, so that when cleaning is required the scale is easily removed once it has been formed. These attempts have been only partially successful since scaling is not completely prevented or minimized nor is the operating time between cleanings substantially extended.
The major scales which can form during the treatment of seawater include calcium carbonate, calcium sulfate and magnesium hydroxide, each presenting different formation prevention and cleaning problems. By way of example, if the treatment device is a spray film vapor compression evaporator operating on seawater, it typically will operate at concentration values of 2.0 or less, while calcium sulfate scale formation usually begins to occur when the concentration factor exceeds about 2.5. However, in badly fouled equipment, some calcium sulfate can deposit, so it cannot be ignored in considering scale inhibitor systems. On the other hand, because most seawater brine discharges in vapor compression distillation equipment have a pH exceeding 9.4, magnesium hydroxide is typically encountered in such equipment along with various forms of calcium, magnesium, carbonate and hydroxide compounds. Calcium carbonate will almost always form in vapor compression equipment along with the magnesium hydroxide and calcium sulfate. They often form complex compounds instead of the pure individual compounds.
It is also known that different scales have different cleaning characteristics, calcium carbonate being the easiest to remove because of its solubility in acid cleaning solutions. Magnesium hydroxide is much more slowly dissolved in acids, while calcium sulfate generally requires a chelating agent for removal.
Factors which affect the rate and amount of scale formation include the initial concentrations of the ionic species in the feed which combine to form the scale and the concentration factor of the brine, the pH of the solution being treated, temperature, ionic strength, the degree of supersaturation at various points in the treatment process, the effectiveness of the scale inhibitor used and the presense of nucleating sites in the water and/or on the surfaces of the equipment where scale crystals have already formed.
The pH of the system can be affected by the scale inhibitor, the initial concentration of the alkalinity and carbon dioxide concentration, the degree of carbon dioxide stripping in the treatment equipment, any thermal decomposition of alkalinity which may occur during treatment and by the ionic strength of the feed water and the ionic strength of the water as treatment proceeds.
The mere presence of scale forming compounds in a feed water does not necessarily mean scale will form, the condition precedent being that the concentration of the component has to exceed its respective saturation value. The point of saturation for each compound is known as the solubility product constant which varies for each, depending on the ionic strength and temperatures of the feed water. In working with scale inhibitor systems, it must also be kept in mind that when comparing the solubility product constant of various compounds, such as calcium carbonate and magnesium hydroxide, it is necessary to determine the actual concentration of the scale inducing ions in solution, even though they are almost never at a stoichiometric ratio. Accordingly, an ion product of the concentrations is used which is the product of the molar concentrations of the cationic and anionic species. When the root of the ion product is compared to the root of the solubility product constant, the degree of supersaturation can be determined. If the ion product is higher, the solution is supersaturated, and the higher it is, the greater the rate of scale formation. Scale inhibitors known prior to the present invention have not been able to prevent scale when the comparison yields a number which is in the range of 3-10 times the saturation factor.
Threshold scale inhibiting (TSI) compounds have been known for many years, and the term is used to describe compounds which can inhibit scale formation when the inhibitor is added in small quantities compared to the quantity of the scale forming salts. For example, certain carboxylic organic acids or salts can tie up calcium or magnesium ions in the feed water. Hydroxy acids, such as citric acid, weakly tie up the hardness ions, while chelating salts, e.g. EDTA, do a more effective job of tying up such ions. The major drawback of such scale inhibitors are that they act stoichiometrically, and therefore must be added in large quantities. The compounds then do not fit the classic definitions of being TSI.
To the knowledge of the present inventor, the first true TSI compounds were inorganic polyphosphates. As little as 10 to 25 ppm of such compounds can suppress the precipitation of from 100 to 300 ppm of calcium carbonate. Polyacrylic and polymaleic acids were also found to posess TSI properties, as were organic phosphonates such as HEDP (1-hydroxyethane-1, 1-diphosphonic acid) and various organic aminophosphonates. The latter presented somewhat of a mystery to researchers in the field, because they are not polymeric, but it is now believed that in waters containing scale forming ions, they do indeed form polymers with the hardness ions acting as links in the polymer formation process. If, however, the feed waters contain iron or other suspended solids, the organic phosphonates may be less effective, probably because of a blockage of the polymerization by the iron salts or the suspended solids.
To be effective as TSI compounds, several factors must be considered. They include the hydrophilic and hydrophobic character of the groups on the polymer and the length of the polymer, which will in term affect the solubility of the polymer in the feed water. Carboxyl, phosphoryl and sulfonic groups in the acid or sodium form add hydrophilic properties, while the same compounds in the calcium or magnesium salt form are hydrophobic. Solubility must also be considered, in that the chemical must be soluble in one form for addition to the treatment water and insoluble in another form after it has abstracted hardness ions from solution. Most TSI polymers have molecular weights in the range of 1,000-10,000, higher molecular weights being insoluble or not sufficiently soluble in the water to be effective.
In addition to the aforementioned characteristics, it is necessary that the ion exchange sites of the TSI material be spaced properly along the chain length so that the valence sites can be tied up by the hardness ions. The TSI compound must remain in suspension until it contacts a crystalline surface, at which time the Helmholtz double layer surrounding the polymer must contract to allow bonding to the crystalline surface. At the same time, the spacing of the ion exchange sites must match the spacing of the ions encountered in the crystalline structure, and perhaps most importantly, the TSI compound must bond with the crystalline surface at the nucleation sites in such a manner that the growth of additional scale is inhibited. Typically, this is accomplished by the production of a hydrophobic surface layer that prevents the deposition of any scale at the nucleation site and distorts any additional scale that does form so that it can be easily removed. Active sites on the crystals thereby become deactivated.
Using calcium carbonate as an example, crystals thereof which are found in water and on equipment surfaces have a relatively small number of nucleation sites for a comparatively large surface area, and a small quantity of the TSI material can deactivate them. The result is that any additional scale that does grow is widely spaced, porous, fragmented and prone to flaking off. Such scale has a relatively large surface area and can then easily be removed by acid cleaning. Absent the use of TSI compounds, the calcium carbonate scale is layered and dense with a relatively small surface area and is much more difficult to dissolve in acid cleaning solutions.
Prior researchers in the field have recognized for some time that a variety of scaling salts exist in water to be treated and that each salt has a different crystal spacing. If only a single scaling salt existed, it would be relatively easy to design a perfect TSI compound for the salt. It has also been suggested that by combining TSI polymers, an improvement may result. It has been hoped that such combinations will provide a synergistic result; i.e that a mixture of two compounds will provide better performance than a simple additive effect. While some synergistic mixtures have been reported, the majority of mixtures in fact produce results worse than the predicted additive effect. One theory used to explain the success of some mixtures is that a multi-dimensional polymer network is formed which is bonded together by the hardness ions. It has also been suggested that synergism for the same reason can result from the use of organophosphonates and a TSI polymer, especially those of the long chain type, such as maleic acid.
Several examples of prior attempts to provide effective TSI compounds or mixtures thereof will be discussed, reference being made to a number of United States patents. Perhaps the very large number of patents involved in the discussion will serve to point out the difficulty in obtaining truly effective and economic TSI products for a variety of uses, from bottle washing equipment to distillation equipment.
In U.S. Pat. No, 3,699,048 issued Oct. 17, 1972 to Krueger, et al., for "Process of Preventing Scale and Deposit Formation In Aqueous Systems and Product", the problem discussed relates to the removal of hardness causing ions in bottle cleaning solutions. The proposed mixture includes an amino alkylene phosphonic acid mixed with an acrylic or methacrylic acid polymer or copolymer, or a copolymer thereof with an ethylenically unsaturated compound, or a graft copolymer of a polysaccharide (e.g. starch), preferably in stoichiometric amounts calculated for the specific hardness system to be encountered. Synergism is claimed, and maleic acid is mentioned as one of the possible unsaturated acids for use in the graft copolymer embodiment.
A different proposal is set forth in U.S. Pat. No. 3,751,372 issued Aug. 7, 1973 to Zecher et al. for "Scale and Corrosion Control in Circulating Water Using Polyphosphates and Organophophonic Acids". The alleged synergistic mixture of the patent is a combination of a water soluble organophosphonic acid, preferably 1-hydroxy alkylene, 1-1-diphosphonic acid with either certain inorganic polyphosphates (or their corresponding acids) or polyfunctional acid phosphate esters of a polyhydric alcohol. The application mentioned in this patent is scale prevention in the circulating cooling water of evaporators.
In U.S. Pat. No. 3,481,869 issued Dec. 2, 1969 to Jones for "Inhibiting Scale", a heavy scale inhibitor is formed by combining a high-density, water solution of amino phosphonic acid and tetra-potassium pyrophosphate. The combination is discussed as being especially useful in preventing scale formation in wells, such as oil wells, even at very low temperatures.
A system for "Scale Inhibition and Removal in Steam Generation" is described is U.S. Pat. No. 3,549,538 issued Dec. 22, 1970 to Jacklin. Boiler feed water is treated with a mixture of two components, the first being a nitrilo compound and the second being a water soluble sulfoxy free polar addition polymer having a molecular weight of 1,000 or more. In the prior art section of this patent, Jacklin describes the attempted use of vinyl-addition polymers of maleic acid (as disclosed in Johnson U.S. Pat. No. 2,723,956), including the preferred styrene-maleic copolymer prepared by reaction of stoichiometric quantities of styrene and maleic anhydride.
In U.S. Pat. No. 3,451,939, issued June 24, 1969 to Palsion, et al., for "Threshold Compositions and Methods", the synergistic compositions include mixtures of polyphosphates and methylene phosphonates.
A further system for "Inhibiting Scale Deposition" is set forth in Ralston's U.S. Pat. No. 3,663,448 issued May 16, 1972. This system includes aminophosphonates mixed with water soluble polyacrylic acid derivatives.
In U.S. Pat. No. 3,505,238 issued Apr. 7, 1970 to Liddell for "Methods and Compositions for Inhibiting Scale in Saline Evaporators", the proposed treating agent is a salt of an amino-methylene phosphonate. A number of additives are suggested including anti-foam agents, water-soluble polymers, tannins, lignins and deareating materials, all designed to inhibit scale formation. The preferred water-soluble polymers are polyacrylamides.
In U.S. Pat. No. 3,682,224 issued Aug. 8, 1972 to Bleyle for "Scale Prevention Agents of Methacrylic Acid--Vinyl Sulfonate Copolymers for Saline Waters Evaporation", the aformentioned combination, with the sulfonate preferably being in a basic salt form, is used to allow the temperature of saline or seawater evaporators to be increased to temperatures as high as 280.degree. F. without precipitation of calcium sulfate. The material is used in concentrations as low as 10 ppm.
Another "Scale Inhibiting Process" is disclosed in the Markofsky et al. U.S. Pat. No. 4,001,134 issued Jan. 4, 1977. Threshold quantities of copolymers of maleic anhydride and allylacetate are used to inhibit scale deposition in seawater distillation plants. This patent also describes the prior art use of maleic acid-styrene copolymers and other maleic anhydride copolymers, including ones capped with polyethylene glycols.
In U.S. Pat. No. 4,147,627 issued Apr. 3, 1979 to Goodman for "Process for Scale Control Using Mixtures of Polycationic and Polyanionic Polymers", the principal problem being addressed is prevention of magnesium hydroxide scale formation in evaporative desalination units. The polyanionic polymers include such materials as acrylic acid polymers, polyamides and polynitriles, while the polycationic materials are complex materials selected from four categories described at columns 3-4 of this patent. This patent is owned by American Cyanamid Company. The American Cyanamid Company also owns another patent related to magnesium hydroxide scale prevention, i.e. U.S. Pat. No. 4,166,041 issued to Goodman on Aug. 28, 1979 for "Process for Magnesium Scale Control Using Mixtures of Polycationic and Polyanionic Polymers". The polyanionic polymer used in this patent is derived from an ethylenically unsaturated dibasic acid (e.g. maleic) or an ethylenically unsaturated sulfonic acid. The polycationic material is selected from the four categories mentioned in the Goodman patent.
Another patent owned by American Cyanimid Company issued to Schiller et al., on Aug. 3, 1982, i.e., U.S. Pat. No. 3,342,652 for "Process for Scale Inhibition in Evaporative Desalination Units". In the patent a copolymer of maleic acid or anhydride and allyl sulfonic acid is used to prevent scale formation in evaporative desalination units.
Walinsky, in his U.S. Pat. No., 4,390,670, issued June 29, 1983 for "Acrylate/Maleate Copolymers, Their Preparation and Use as Antiscalants" describes a particular system for the preparation of the copolymer so that monomer and polymer species remain in solution. Use thereof at levels of 0.1 to 100 ppm is stated to prevent calcium and magnesium scale formation. The same inventor was issued U.S. Pat. No. 4,485,223 on Nov. 27, 1984 for "(Meth)acrylic Acid/Itaconic Acid Copolymers, Their Preparation and Use as Antiscalants." The patent describes a particular way of adding the monomers and inhibitors. Finally, U.S. Pat. No. 4,547,559 was issued to the same inventor on Oct. 15, 1985 for "Acrylate/Maleate Copolymers, Their Preparation and Use as Antiscalents." This patent is similar to the first mentioned Walinsky patent.
While the aforementioned patents describe a wide variety of antiscalants, scale formation in desalination units and other devices where the feed water contains scale forming ions is still a problem. While the various chemical systems mentioned above provide benefits which are significant when compared to untreated feed streams, the discovery of an antiscalant system which is substantially better than commercial products and the above suggested systems would represent an important advance in the art.